In supersonic inlets of aircraft power plants, a normal shock must be maintained to decelerate the inflowing captured airstream before it enters the engine This is necessary in order to maintain stable operation of the engine. Various types of inlet control systems are known for adjusting the inlet to maintain the normal shock in an acceptable position in the inlet. The particular desired shock location and the range of movement of the shock permissible depends on the particular type of engine. In order to maintain the shock in the required position, the inlet control system must have accurate input representing the current position of the shock.
Since the location of the normal shock is very sensitive to the free stream Mach number, altitude, vehicle speed, engine flow rate, and other factors, it must be continually monitored to provide feedback to the inlet control system. Most prior approaches to solving the problem of providing accurate measurement and communication of the shock location have been based on the effect of the normal shock on static pressure. There is a sudden increase in static pressure which occurs as the incoming flow in the inlet crosses the normal shock. The approach of basing the shock location measurement on the detection of this increase in static pressure is complicated by the presence of small oblique shocks along the inlet walls. The pressure changes from these relatively minor shocks are detected as noise which limits the resolution of the measurement of the normal shook location to about an inch. In many situations, a much higher degree of accuracy is desired.
U.S. Pat. No. 2,971,330, granted Feb. 14, 1961, to J. W. Clark, discloses a sound shook locator and control system for positioning the shock in high speed air inlets for aircraft power plants. In the disclosed system, a sound generator and a receiver are placed in the inlet in the flow path downstream of the shock location. The sound generator directs a high frequency sound upstream toward the normal shock. The receiver senses the echo of the sound reflected by the shock. Signals from the generator and receiver are processed by an acoustic system which sends a resultant signal to an inlet geometry control for adjusting the position of the shock.
U.S. Pat. No. 3,623,361, granted Nov. 30, 1971, to B. H. Funk, Jr., discloses a method and apparatus for measuring the statistical properties of turbulence in supersonic flows. The system measures both translational and rotational aspects of the turbulent motion. The method includes projecting a first beam of collimated light and detecting deflections in the beam, projecting a second beam of collimated light and detecting deflections in the second beam, and correlating the characteristics of the deflections of the first and second beams to determine the characteristics of the turbulence. The apparatus for detecting the deflection of each beam includes a photodetector with a knife edge positioned in front of it to cut off about 50 percent of the light.
U.S. Pat. No. 3,714,827, granted Feb. 6, 1973, to C. N. Batts, discloses a measurement circuit to obtain buffet data from wind tunnel models. The information is used in the design of aerodynamic vehicles.